Cellular radio systems are currently in widespread use throughout the world providing telecommunications to mobile users. Available frequency bands for mobile communications are divided up into a number of carrier frequencies. In order to meet the demand for transmission capacity within an available frequency band allocation, cellular radio systems divide a geographic area to be covered into a plurality of cell areas. Each cell is provided with a base station with which a plurality of mobile stations within the cell communicate.
In general, an object of cellular radio communication system design is to have as few base stations as possible, since base stations are expensive, and require extensive effort in obtaining planning permission and in some areas, suitable base station sites may not be available.
In order to have as few base stations as possible, each base station ideally has as large a capacity as possible so as to service as large a number of mobile stations as possible. However, there are fundamental limits on the maximum user capacity of a base station for serving mobile stations because the number of mobile stations which can be served by a base station depends upon the available number of carrier frequencies, and these carrier frequencies are a limited resource in the frequency spectrum. In order to physically separate radiations at the same or closely matching frequencies, it is known to make use of directional antennas which produce directional radiation beams on the downlink (base station to mobile communication). The use of directional radiation beams allows greater reuse of carrier frequencies, and increases system capacity compared with omni-directional antennas.
The type of antenna used at the base station site can potentially make significant improvements to the capacity of a cellular radio system. The conventional approach is to use omni-directional antennas, broadly directional tri-sectored antennas or hex-sectored antennas.
The use of directional antennas in current cellular radio systems is based upon the principle of sectorization, as illustrated in FIG. 1 herein which shows schematically an area covered by a cell pattern in which a plurality of nominally hexagonal cells in a N=7 cluster size hexagonal pattern are each divided into three sectors, each spanning a range of 120.degree. azimuth. The main source of interference comes from mobile stations in the so called first tier reuse cells 100-105, which in the example of FIG. 1 are spaced apart from a center cell 106 by a distance of at least two intermediate cells, for example, cells 107, 108. Using omni-directional antennas, the base station antenna in center cell 106 receives interference from mobile stations in all six first tier reuse cells 100-105. However, if an antenna has a nominal 120.degree. beam width corresponding to a cell sectorized into three sectors (a tri-sectorized configuration) interference is received from mobile stations in only two first tier re-use cells, 100, 101. The situation can be improved by using an antenna with a 60.degree. beam width, corresponding to a hex-sectored configuration, in which case interference is received from mobiles in only one of the first tier cells.
One previously suggested approach to increase system capacity by increasing frequency re-use is that of a base station antenna pattern comprising beams of narrow angular widths in both azimuth and elevation as shown in FIGS. 2 and 3 herein. Prior art literature recognizes many of the potential benefits of narrow beam antennas.sup.1, 2. Although a narrow radiation beam is formed at radio frequencies typically in the 900 MHz, 1800 MHz or 1900 MHz bands, a narrow beam can usefully be visualized as analogous to search light beams which emanate from the base station, and track the mobiles. When contrasted with an omni-directional tri-sectored antenna, this creates a high quality transmission path with minimal interference. A plurality of such narrow beams are provided in each 120.degree. sector. A narrow radiation beam 200 may be directed by a base station smart antenna 201 at a desired mobile station 102, and tracks the mobile's movements. When compared to an omni-directional antenna, such a narrow beam has dual benefits of having high gain, leading to increased range in a thermal noise limited environment, and of rejecting interference from cells employing co-frequency reuse, due to spatial separation of beams, thereby allowing higher capacity in a cell without cell splitting. A capacity increase is obtained by tighter frequency reuse across the network. For the purposes of this document, the use of the word "omni-directional" is intended to convey the meaning of having radiation coverage over an area corresponding to substantially the whole geographic area of a cell.
Where each cell has a number of narrow smart antennas having narrow beams which track individual mobiles, there results a reduction in overall carrier to interference (C/I) ratio due to the statistical probability that different beams re-using the same carrier frequency will be pointing in different directions, having different azimuth and different elevations. The randomized locations of mobiles (and hence beam direction) means that there is a low likelihood of intercept interference, and the likelihood of two or more beams of a same carrier frequency intercepting each other is diminished. The narrower the beams, the lower the probability that a mobile will intercept a same frequency beam of a different cell in which the carrier frequency is re-used.
The extent of the advantage of a narrow beam antenna over an omni-directional antenna is a function of the beam width of the narrow beam antenna, and the narrower the beam width, the greater the advantage in terms of C/I performance. However, narrow beam antennas have increased size and complexity compared with omni-directional antennas and there are key disadvantages in the approach of using a large number of narrow directional antennas in a sectorized approach.
The cellular radio transceivers are dedicated to particular sectors, which leads to significant levels of trunking inefficiency. In practice, this means that many more transceivers are needed at the base station site than for an omni-directional cell of the same capacity.
Each sector is treated by the cellular radio network (i.e. the base station controller and mobile switches) as a separate cell. This means that as a mobile moves between sectors, a considerable interaction is required between the base station and the network in order to hand off the call between sectors of the same base station. This inter-action comprises signaling and processing at the base station controller and switch and represents a high overhead on the network, and reduces capacity.
The problem can be illustrated by considering the operations necessary or frequency allocation in relation to a structure of a base station. Referring to FIG. 4 herein, a conventional cellular radio system comprises several layers including a mobile switching center (MSC) 400 which provides an interface between the cellular system and other networks, for example, the public switched telephone network (PSTN), or integrated service digital network (ISDN) 401. Each mobile switching center 400 controls several base station systems (BSS) 402-404 which in some systems such as the Groupe Systeme Mobile (GSM) or PCS 1900 systems are further divided into a base station controller (BSC) 405 which controls several base transceiver stations (BTS) 406-408. Each base transceiver station communicates with several mobile stations (MS) 409-411. At the mobile switching center level, there are also other facilities such as operations and maintenance controller (OMC) 412 and network management controller (NMC) 413 apparatus. In the conventional cellular radio system, calls are allocated to transceivers at base band in the cellular radio network, at either the base station controller if available, or at the mobile station controller, as shown in FIG. 4 herein. Any change required in handing over a call to a different transceiver has to be signaled through the network, possibly as far as the mobile station controller, and back again.
In some cases, the approach of using a large number of narrow beams to increase system capacity encounters further problems to those identified above. In particular, as described later in this document in the North American Digital-AMPS system the beams must be spatially fixed, and cannot be steered to follow a mobile station. Using a large number of directional beams under these circumstances introduces additional problems in trunking inefficiency.